In warehouses, goods are normally unloaded and placed into pallet racks until needed for selling or further processing. When an order arrives, a stacker, or staple, truck is sent out to pick up the goods and deliver them for further transport or processing. The handling of goods in this way is very labor intensive, since every delivery and pick up order demands the operation of one truck and one driver.
In order to increase the operation efficiency of goods handling in warehouses, industrial trucks with double loading capacity have been developed. Such industrial trucks, generally known as double staple or stacker trucks, are provided with two pair of forks. In operation, the truck loads one pallet on the first forks and raises the pallet on the mast. Thereafter another pallet may be loaded on the second forks. The two pallets may thereafter be transported to a loading/unloading platform. A double stacker industrial truck according to the preamble of claim 1 is described in DE 20 2005 015 354 U1. In this known truck, both load carriages are arranged on the same mast and are driven separately in the vertical direction.
One problem with such a construction is that when both load carriers are moved on the mast, only the load carrier with the lowest hydraulic pressure will move. The difference in hydraulic pressure is normally caused by variations in load or friction between the load carriages and the mast and could cause the loads to collide on the mast. The separate drives of the load carriers also makes it difficult to equally divide the lifting work between the load carriers when both carriers are used to lift one single load.
It is an object of the present invention to provide a hydraulic system for a double stacker industrial truck which solves the aforementioned problem. A further object is to provide a hydraulic system which provides greater flexibility between different lifting operations. A further object is to provide a double stacker industrial truck comprising a hydraulic system which solves the aforementioned problem. Yet a further object is to provide a double stacker industrial truck which provides greater flexibility between different lifting operations.